This invention relates to geometrical instruments and particularly to an instrument for measuring the location and orientation of openings in a casting and particularly of a plurality of ports spaced around a tubular cast compressor body to determine any shift or deviation in each port from a nominal port location.
Large scale industrial compressors are commonly constructed with a generally tubular shaped body defining a cylinder in which a piston reciprocates. The tubular compressor body may form a cylinder as much as two feet or more in diameter and may have an axial dimension several feet in length to provide an extremely large displacement for the reciprocating piston. The walls of the compressor body are several inches thick and are provided with a plurality of ports which may be spaced both axially and circumferentially thereabout for mounting intake and exhaust valves.
The compressor body is initially cast from metal in a mold having cores forming the main cylinder in which the piston reciprocates and forming the various ports in which the intake and exhaust valves are mounted. The ports are smaller than the final port openings to permit machining these openings to precise dimensions. Nowadays, the final machining is commonly accomplished on an automatic machine tool which is under the control of a computer. The computer is programmed to automatically control the machine tool for each machining phase at each valve port, which includes milling, rough boring, finishing and drilling bolt holes for attachment of exterior manifolding. Passages are also provided within the compressor body for circulation of a coolant. In order to achieve a maximum and uniform cooling, only a limited amount of metal stock is allowed in the walls of each valve port. Normal manufacturing tolerances and also non-uniform cooling of the cast compressor body can cause the valve ports to shift from their desired or nominal location during the casting process. The shifting may take several forms. For example, the entire port may shift by a small angular amount about the axis of the compressor body or it may shift in an axial direction. Or, the port at either or both the interior or exterior surface of the compressor body may shift by different amounts or in different directions so that the axis of the cast port is skewed and out of alignment with the axis of the main cylinder in which the compressor piston reciprocates. Since only a limited amount of metal stock is allowed in the walls of each valve port, a defective compressor body may be produced if the valve port is machined at its nominal location without taking into account the shift or deviation of the port from the nominal location during the casting process. Therefore, it is desirable to measure the orientation and location of each port in a cast compressor body and to supply this data to the computer controlled machine tool to insure optimal symmetry for equal stock distribution about the walls of each valve port or pocket after machining.